U.S. patent application number 10/815104 was filed with the patent office on 2005-08-25 for method for determining a rotor position of a synchronous motor.
This patent application is currently assigned to SIEMENS AKTIENGESELLSCHAFT. Invention is credited to Kunzel, Stefan, Nemeth-Csoka, Mihaly.
Application Number | 20050187735 10/815104 |
Document ID | / |
Family ID | 34853546 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050187735 |
Kind Code |
A1 |
Nemeth-Csoka, Mihaly ; et
al. |
August 25, 2005 |
Method for determining a rotor position of a synchronous motor
Abstract
In a method for determining the rotor position of a synchronous
motor, a plurality of current vectors having different directions
is applied to the synchronous motor. Absolute values of required
ones of the current vectors are determined to obtain a defined
excursion of the rotor. Inverse values of the determined current
vectors are then digitally filtered using several of the inverse
values to determine Fourier coefficients of the first harmonic of
the inverse values. The rotor position of the synchronous motor can
then accurately be computed with the determined Fourier
coefficients.
Inventors: |
Nemeth-Csoka, Mihaly;
(Erlangen, DE) ; Kunzel, Stefan; (Erlangen,
DE) |
Correspondence
Address: |
HENRY M FEIEREISEN, LLC
350 FIFTH AVENUE
SUITE 4714
NEW YORK
NY
10118
US
|
Assignee: |
SIEMENS AKTIENGESELLSCHAFT
Munchen
DE
|
Family ID: |
34853546 |
Appl. No.: |
10/815104 |
Filed: |
March 31, 2004 |
Current U.S.
Class: |
702/151 |
Current CPC
Class: |
H02P 25/024 20160201;
H02P 21/22 20160201 |
Class at
Publication: |
702/151 |
International
Class: |
G01C 019/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 19, 2004 |
DE |
10 2004 008 250.2 |
Claims
What is claimed is:
1. A method for determining a rotor position of a synchronous
motor, comprising the steps of: applying to a synchronous motor a
plurality of current vectors having different directions;
determining absolute values of required ones of the current vectors
to obtain a defined excursion of the rotor; determining inverse
values of the determined absolute values of the current vectors;
digitally filtering using several of the inverse values to
determine Fourier coefficients of a first harmonic of the
determined inverse values; and computing with the determined
Fourier coefficients the rotor position of the synchronous
motor.
2. The method of claim 1, wherein an opposite mathematical sign is
applied to the determined inverse values in a range of a negative
excursion of the rotor before determining the Fourier
coefficients.
3. The method of claim 2, wherein the Fourier coefficients of the
first harmonic of the determined inverse values are determined by
digitally filtering using all inverse values.
4. The method of claim 2, wherein the Fourier coefficients of the
first harmonic of the inverse values are determined by digitally
filtering only the inverse values that are in the range of the
negative excursion of the rotor.
5. The method of claim 1, wherein the Fourier coefficients of the
first harmonic of the inverse values are determined by digitally
filtering using several inverse values only in a range of a
positive excursion of the rotor.
6. The method of claim 1, and further comprising the step of
applying a brake to hold the rotor before applying the plurality of
current vectors.
7. A data carrier having a computer program stored thereon for
carrying out the method of claim 1.
8. A computer with a program memory having stored therein a
computer program, said computer program causing the computer to
execute the method of claim 1.
9. The computer of claim 8, wherein the computer comprises a
controller.
10. A machine-tool or production machine comprising a controller
according to claim 9.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the priority of German Patent
Application, Ser. No. 10 2004 008 250.2, filed Feb. 19, 2004,
pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is
incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] The present invention relates to a method for determining a
rotor position of a synchronous motor. The present invention also
relates to a data carrier with a computer program stored thereon
for performing method, as well as to a computer with a program
memory for storing the computer program. The present invention
further relates to a machine-tool or production machine with the
computer, wherein the computer can be implemented as a controller.
In the following description, the term "production machine" is used
in a generic sense and also includes robots which generally follow
the concepts outlined here.
[0003] Several conventional methods are known for determining the
position of a rotor of a synchronous motor relative to the stator
of the synchronous motor in order to determine the electrical
commutation of the motor without requiring a position measuring
device to provide position information. Many of these methods are
based on complex measurements of the inductances of the motor, or
on measuring and determining the electromotive counterforce. Such
methods make it possible to operate a synchronous motor without
employing a position measuring device.
[0004] If the rotor position angle is only determined during
ramp-up or start-up of the synchronous motor, for example for
determining an offset between the zero position of the position
measuring device and the rotor or for moving the rotor to a
reference point, then less complex methods can be used to determine
the rotor position.
[0005] German patent publication DE 102 15 428 A1 describes a
method for determining the rotor position of a synchronous motor,
whereby a plurality of current vectors is applied to the
synchronous motor in different directions and the absolute value of
the current vector necessary for obtaining a defined excursion of
the rotor is determined. The motor position is subsequently
computed from at least one angular position of the current vector,
at which the absolute value of the current vector necessary for
achieving the defined excursion is at a minimum. However, the curve
describing the absolute value of the current vectors is very flat
particularly in the vicinity of the minimum absolute values of the
current vectors and the measured values have superimposed
disturbances in practical applications. As a consequence, the motor
position cannot be adequately determined.
[0006] It would therefore be desirable and advantageous to provide
a method for determining the rotor position of a synchronous motor,
which obviates prior art shortcomings and is specifically capable
of accurately determining the rotor position in spite of existing
disturbances and the flatness of the current vector curve.
SUMMARY OF THE INVENTION
[0007] According to one aspect of the present invention, a method
for determining the rotor position of a synchronous motor includes
the steps of applying to a synchronous motor a plurality of current
vectors having different directions, determining absolute values of
required ones of the current vectors to obtain a defined excursion
of the rotor, determining inverse values of the determined absolute
values of the current vectors, digitally filtering using several of
the inverse values to determine Fourier coefficients of a first
harmonic of the determined inverse values, and computing with the
determined Fourier coefficients the rotor position of the
synchronous motor.
[0008] According to another advantageous feature of the invention,
an opposite mathematical sign can be applied to the determined
inverse values in a range of where the excursion of the rotor is
negative, before determining the Fourier coefficients. In this way,
the values of the current vectors that are measured in the range of
a negative excursion of the rotor can also be used for determining
the rotor position of the synchronous motor.
[0009] According to another advantageous feature of the invention,
the motor position can be measured with particular accuracy by
determining the Fourier coefficients of the first harmonic of the
inverse values through a digital filtering process using all
inverse values.
[0010] According to still another advantageous feature of the
invention, the Fourier coefficients of the first harmonic of the
inverse values can be determined by digitally filtering only the
inverse values that are in a range where the excursion of the rotor
is negative. Alternatively, the Fourier coefficients of the first
harmonic of the inverse values can be determined by digitally
filtering several inverse values only in a range where the
excursion of the rotor is positive. These approaches minimize the
computing time as compared to an evaluation process that includes
all the inverse values.
[0011] According to another advantageous feature of the invention,
a brake can be applied that holds the rotor before applying the
plurality of current vectors. By using a brake, the method can be
performed independently of the mechanical configuration of the
synchronous motor and of other mechanical conditions.
[0012] According to another aspect of the invention, the method can
be executed by providing a data carrier with the computer program
stored thereon. In addition, a computer can be provided with a
program memory for storing a computer program that causes the
computer to execute the afore-described method. Advantageously, the
computer can be implemented as a controller.
BRIEF DESCRIPTION OF THE DRAWING
[0013] Other features and advantages of the present invention will
be more readily apparent upon reading the following description of
currently preferred exemplified embodiments of the invention with
reference to the accompanying drawing, in which:
[0014] FIG. 1 is a block diagram of a drive unit with a synchronous
motor;
[0015] FIG. 2 is a vector diagram of the rotor or stator
currents;
[0016] FIG. 3 is a graphical illustration showing the absolute
values of the current vectors as a function of the rotor position
angle;
[0017] FIG. 4 is a graphical illustration showing the absolute
values of the current vectors, the corresponding inverse values and
the excursion as a function of the rotor position angle; and
[0018] FIG. 5 is a schematic representation of a machine-tool or
production machine.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
[0019] Throughout all the Figures, same or corresponding elements
are generally indicated by same reference numerals. These depicted
embodiments are to be understood as illustrative of the invention
and not as limiting in any way. It should also be understood that
the drawings are not necessarily to scale and that the embodiments
are sometimes illustrated by graphic symbols, phantom lines,
diagrammatic representations and fragmentary views. In certain
instances, details which are not necessary for an understanding of
the present invention or which render other details difficult to
perceive may have been omitted.
[0020] Turning now to the drawing, and in particular to FIG. 1,
there is shown a block diagram of a drive unit with a synchronous
motor 2, which is controlled by a controller 1. The rotor position
of the synchronous motor 2, which in the illustrated exemplary
embodiment is defined as a rotor position angle .rho., is measured
by a position measuring device 3. The position measuring device 3
can be implemented, for example, in the form of an incremental
rotation encoder enables computation of absolute position values
only after moving past a reference point. The position measuring
device 3 is connected with the shaft 4 of the synchronous motor 2.
Since the shaft 4 is rigidly connected with the rotor of the
synchronous motor 2, the rotor position angle .rho. defines the
position of the shaft 4. Optionally, a brake 5 can be applied to
the shaft 4. Such mechanical or electrical brakes 5 are commonly
used to quickly brake a motion or to hold axes without applying an
electrical current. The brake 5 can be activated by the controller
1. A useful load 6 can be driven by the shaft 4, for example, the
tool spindle of a machine-tool or production machine, or a linear
axle of a production machine driven by the spindle. The drive unit
can also be used for driving a production machine or any other
suitable type of machine.
[0021] FIG. 2 shows in form of a vector diagram the effective
directions of the magnetic fields and the applied current vector I.
The magnetic field of the rotor of a permanent-excited synchronous
machine has a major effective direction d which rotates together
with the rotor. The magnetic field is generated, for example, by
permanent magnets disposed of the rotor. A force can be generated
by applying the current vector I, which can have the form of a
current space vector, to the stator windings in a direction q a
perpendicular to the magnetic field. Since the magnetic induction
is fixedly defined by the permanent magnets of the rotors, a
field-oriented control requires that information about the actual
rotor position angle .rho. is available at each point in time. The
machines typically include a position measuring device in the form
of a transducer with incremental marks or tracks. Because the
transducer does not have an absolute reference position, an initial
position value .rho..sub.0 of the rotor position angle .rho. has to
be determined before a controlled startup of the motor.
[0022] A 360.degree. rotation in the vector diagram of FIG. 2
corresponds to an electric 360.degree. rotation. In a multi-pole
machine, a mechanical 360.degree. rotation can correspond to
several electric 360.degree. revolutions. It should be noted that
all described angles relate to the electric rotation. It will also
be understood, that a synchronous motor can be implemented not only
as a rotary motor, but also as a linear motor, in which case the
angles are converted to distances on a linear axis. It is not
important for the described method of the invention if the
permanent magnets are applied on the rotor side, as in the depicted
example, or on the stator side, in which case the current vector
would be applied to the rotor.
[0023] The rotor position angle .rho..sub.0 of the stationery motor
will now be determined by the method of the invention. With the
brake 5 (see FIG. 1) applied, several current vectors I with
different angular positions (p (see FIG. 2) are applied to the
synchronous motor 2. A current deviation having an absolute value I
parallel to the current vector I is applied to each of these
current vectors I in order to generate a small defined excursion
.DELTA..beta. of the rotor against the holding force of the brake.
The resulting curve depicted in FIG. 3 shows two minima for the
values I of the current vectors I that are each offset from each
other by 90.degree. and located before and after the rotor position
angle .rho..sub.0 that is to be determined. The excursion
.DELTA..beta. of the rotor changes its direction from negative
values to positive values and in the vicinity of the position of
the rotor position angle .rho..sub.0 to be determined.
[0024] The current vector I is hereby defined by the equation:
I(.phi.-.rho..sub.0)=I.multidot.sin(.phi.-.rho..sub.0) (0)
[0025] The torque of a synchronous motor in the voltage control
range is proportional to the absolute value I of the current and
the sine of the angle .phi. of the current vector with respect to
the major effective direction d of the magnetic field of the rotor
according to FIG. 1. The torque of the synchronous machine is
then:
m.sub.1=k.sub.1.multidot.I.multidot.sin(.phi.-.rho..sub.0) (1)
[0026] wherein m.sub.1 is the torque and k.sub.1 is the motor
constant which is derived from the mechanical and electrical
configuration of the motor.
[0027] The return torque m.sub.2 of the braked axle is defined by
the equation:
m.sub.2=k.sub.2.multidot..DELTA..beta. (2)
[0028] wherein k.sub.2 is the spring constant and .DELTA..beta. the
excursion measured by the position measuring device.
[0029] In the steady-state, the two torques are identical, so that
the following equation governs:
k.sub.1.multidot.I.multidot.sin(.phi.-.rho..sub.0)=k.sub.2.multidot..DELTA-
..beta. (3)
[0030] Following the basic idea of the measurement, the respective
absolute value I of each current vector I is increased until the
excursion .DELTA..beta. reaches a predefined value. In this
stationery state, the constants k.sub.1, k.sub.2 and .DELTA..beta.
can be combined into a single constant K. The equation (3) can then
be written in a simplified form as: 1 I ( - 0 ) = K 1 sin ( - 0 ) (
4 )
[0031] In FIG. 3, the absolute values I(.phi.-.rho..sub.0) of the
current vectors I are plotted as a function of the angle
.phi.-.rho..sub.0.
[0032] As seen in FIG. 3, evaluating the current minima, as
proposed in German patent publication DE 102 15 428 A1, is
problematic because the curve shape is quite flat about the minimum
position and because the current values are small. In addition, in
an actual measurement the measured values, unlike the depicted
idealized points, frequently have superimposed noise and other
disturbance values, so that the measured minima do not always
coincide with the actual minima. Since a current value may only be
determined every 5.degree., the rotor angle .rho..sub.0 to be
determined can also be located between two adjacent measured
values, which can result in an erroneous determination of the rotor
position angle .rho..sub.0. Averaging these values, as proposed in
DE 102 15 428 A1, does not eliminate the afore-described
disadvantages.
[0033] Unlike the method described in German patent publication DE
102 15 428 A1, the present invention employs an entirely different
evaluation technique.
[0034] FIG. 4 shows the operation of the method of the invention.
Shown in FIG. 4 is again the curve with the absolute values
I(.phi.-.rho..sub.0) of the current vectors I plotted as a function
of the angle .phi.-.rho..sub.0, corresponding to the curve of the
absolute values I of the current vectors I of FIG. 3. According to
the invention, the inverse values of the absolute values of current
vectors are determined in a first method step. The inverse values
1/I(.phi.-.rho.) are obtain from 2 1 I ( - 0 ) = 1 K sin ( - 0 )
for 0 ( 5 ) 1 I ( - 0 ) = - 1 K sin ( - 0 ) for < 0 ( 6 )
[0035] The inverse values of the absolute values of current vectors
have the opposite mathematical sign in those regions where the
excursions .DELTA..beta. are negative. In the illustrated exemplary
embodiment, the excursion .DELTA..beta. is positive in a range
between 45.degree. and 225.degree. and negative in a range between
225.degree. and 45.degree., as shown in FIG. 4. Accordingly, in the
illustrated exemplary embodiment, the inverse values were taken
between 45.degree. and 225.degree., whereas the opposite
mathematical sign was applied to the inverse values taken between
225.degree. and 45.degree.. The inverse values
1/I(.phi.-.rho..sub.0) have then a sinusoidal dependence. In
addition to the curve with the absolute values I(.phi.-.rho..sub.0)
of the current vectors I plotted as a function of the angle
.phi.-.rho..sub.0, FIG. 4 also shows curves with the inverse values
1/I(.phi.-.rho..sub.0) as well as the excursion .DELTA..beta.
plotted as a function of the angle .phi.-.rho..sub.0. The zero
position on the ordinate for each curve is also shown. The upper
zero position is associated with the excursion .DELTA..beta., the
center zero position is associated with the inverse values
1/I(.phi.-.rho..sub.0), and the lower zero position is associated
with the absolute values I(.phi.-.rho..sub.0) of the current
vectors I.
[0036] In the next method step, the Fourier coefficients a and b of
the first harmonic of the inverse values of the absolute values of
the current vectors are determined by digital filtering over
several inverse values. Several digital filtering methods are known
that can be used to determine the Fourier coefficients a and b. In
the present embodiment, a complex Fourier transformation which is
known in the art is used to determine the Fourier coefficients by
digital filtering. The first harmonic of the inverse values of the
absolute values of the current vectors is defined by the equation:
3 1 I ( - 0 ) = a sin ( - 0 ) + b cos ( - 0 ) ( 7 )
[0037] wherein a is the Fourier coefficient of the imaginary part,
and b is the Fourier coefficient of the real part.
[0038] The rotor position angle .rho..sub.0 can then be determined
from the Fourier coefficients a and b via the relationship: 4 0 =
arctan ( a b ) ( 8 )
[0039] The arc tangent function is determined by properly taking
into account the mathematical signs of a and b in all four
quadrants.
[0040] Several possibilities exist for digitally filtering the
inverse values, i.e., for selecting the values and the number of
inverse values to be used for computing the Fourier coefficients.
To obtain the most accurate result, it may be advisable to compute
the Fourier coefficients by including all the inverse values, i.e.,
the width of the digital filtering window includes all the inverse
values of a complete electrical 360.degree. rotation. If the
available computing time is less, then the rotor position value
.rho..sub.0 can be determined with sufficient accuracy by selecting
for determining the Fourier coefficient only those inverse values
where the excursion .DELTA..beta. is positive. Alternatively, only
inverse values associated with negative values of the excursion
.DELTA..beta. may be included.
[0041] It will be understood by persons skilled in the art that
subsets of the afore-described inverse values can be used for
determining the Fourier coefficients; alternatively, inverse values
from both ranges can be used for digital filtering.
[0042] The accuracy with which the rotor position angle .rho..sub.0
can be determined depends on the number of inverse values used for
determining the Fourier coefficients, i.e., the determination
becomes more accurate with an increased width of the digital filter
window. However, this also increases the computing time for digital
filtering. In other words, using a large number of measured values
for determining the rotor position angle .rho..sub.0 increases the
accuracy with which the rotor position angle .rho..sub.0 can be
determined.
[0043] A brake does not need to be applied for performing the
method of the invention if the rotor can be held in a stationary or
rest position by a suitable existing return torque, for example a
friction force.
[0044] As shown in FIG. 5, the method of the invention can be
performed with a computer 7 having a program memory 8 in which a
computer program 9 can be stored. The method of the invention can
then be executed by the computer 7 by calling the computer program
9. The computer can be implemented, for example, as a controller.
The controller 7 in the depicted exemplary embodiment of FIG. 5 is
a component of a machine tool or production machine 10, whereby the
machine tool or production machine 10 can include additional
components that have been omitted from the drawing because they are
not relevant for an understanding of the invention.
[0045] The computer program 9 also be stored on a data carrier 11,
whereby the data carrier can be implemented, for example, as a
flash card, diskette, CD-ROM, DVD or a hard drive.
[0046] Machine tools in the context of the present invention can
also include, for example, uniaxial or multi-axis lathes, milling
machines, as well as drilling or grinding machines. Machine tools
can further include processing centers, linear and rotary transfer
machines, laser machines, rolling machines and/or gear cutters.
These machines have in common that the material is machined along
several axes. Production machines in the context of the present
invention can include textile, paper, plastic, wood, glass, ceramic
or stone processing machines, as well as machines used for forming,
packaging, printing, conveying, lifting, pumping, transporting.
Furthermore, fans, blowers, wind turbines, lifting gear, cranes,
robots, production and assembly lines are also included under the
term production machines in the context of the present
invention.
[0047] The method of the invention can also be performed with any
type of synchronous motor and is not limited to applications
associated with machine tools or production machines.
[0048] While the invention has been illustrated and described in
connection with currently preferred embodiments shown and described
in detail, it is not intended to be limited to the details shown
since various modifications and structural changes may be made
without departing in any way from the spirit of the present
invention. The embodiments were chosen and described in order to
best explain the principles of the invention and practical
application to thereby enable a person skilled in the art to best
utilize the invention and various embodiments with various
modifications as are suited to the particular use contemplated.
[0049] What is claimed as new and desired to be protected by
Letters Patent is set forth in the appended claims and includes
equivalents of the elements recited therein:
* * * * *